1. Field of the Invention
The present invention relates to a method of producing a color conversion table for color conversion from the color image signal to be handled in an input system into a color image signal to be handled in an output system, image processing apparatus, method of image processing, image forming apparatus and recording media.
2. Description of Related Art
In recent years, a color printer, color copying machine and combination machine thereof have come into frequent use. In many cases, the color image forming apparatus used in these machines incorporates a 3D color information conversion table (3D Lookup Table; also called the RGB→4 CMYK3D−LUT in the following description) for color conversion from the image information of red (R), green (G) and blue (B) in a signal processing system into the image data of a CMYK signal processing system. This is because the image forming apparatus adopts the structure wherein the operation is performed according to the image data of the CMYK signal processing system.
The RGB→CMYK3D−LUT is created by matrix processing or interpolation arithmetic processing according to the color measurement value of n3 patch documents (XYZ or LAB) wherein patches are arranged so as to increase the intensity of each of the RGB colors, and the scanner signal (RGB). Then the RGB→CMYK3D−LUT is used to convert the RGB signal into the XYZ output signal or Lab output signal.
The Patent Document 1 discloses an image processing apparatus related to the image forming apparatus handling the aforementioned color image data. This image processing apparatus applies processing of color gamut compression to the color reproduction area of an output device for the image data inputted from an input device. The image data subjected to color gamut compression is outputted to the output device.
FIGS. 17A and 17B show the color coordinate system representing an example of the CMY→Lab3D−LUT and RGB→Lab3D−LUT in the conventional Lab color coordinate system.
The vertical axis in FIG. 17A represents a brightness L* axis, and the horizontal axis denotes a chromaticity b* axis. Chromaticity a* axis is found at the intersection between the brightness L* axis and chromaticity b* axis. In FIG. 17A, the first lattice pattern I corresponds to the RGB→Lab3D−LUT H wherein the color image signals R, G and B of the RGB signal processing system is represented in 256 gradations. The contour line of the lattice pattern I shows the color gamut boundary that can be handled by the RGB signal processing system.
In this example, the lattice pattern. II displayed so as to be included in the lattice pattern I corresponds-to the CMY→Lab3D−LUT wherein the color image signals C, M, Y and K handled by the CMYK signal processing system are expressed in 256 gradations. The contour line of the lattice pattern II shows the color gamut boundary (hereinafter also referred to as “printer color gamut”) handled by the CMYK signal processing system. In this displayed example, if that the area inside the printer color gamut is defined as “inside the printer color gamut”, and the area outside the printer color gamut as “outside the printer color gamut”, a decision step is taken to determine if a target for arithmetic processing is located inside or outside the printer color gamut.
The circle showed in a broken line of FIG. 17B is an enlarged view of the area close to the printer color gamut in FIG. 17A. The RGB→Lab3D−LUT shown in FIG. 17B exhibits the case when the lattice point is located across the printer color gamut. In this example, the lattice points a and b are located outside the printer color gamut. The lattice points c and d are found inside the printer color gamut. This is the case where a non-lattice point (arithmetic processing reference point) e is given in the area enclosed by these four lattice points a, b, c and d and outside the printer color gamut. In this case, in the image processing apparatus disclosed on Patent Document 1, when the input RGB value outside the printer color gamut is subjected to color conversion and interpolation as shown in FIG. 17B, then processing of compression is applied by the output CMYK values of four lattice points a, b, c and d, so that interpolation is carried out inside the printer color gamut.
FIG. 18 is a flowchart representing and example of the conventional RGB→CMYK3D−LUT. In the Step F1 of the flowchart shown in FIG. 18, the input data and Lab data are read out from a predetermined image memory and are displayed on the RAM. The input data and Lab data are displayed on the RAM, so that the RGB→Lab3D−LUT is created.
CMY data is read out from the image memory in the Step F2, and is displayed on the RAM. The CMY data is displayed on the RAM, so that the CMYK→4 Lab4D−LUT is created. Then the system goes to the Step F3 and the CMY→Lab3D−LUT is created from the CMYK→Lab4D−LUT, based on the GCR algorithm.
After that, the system goes to the Step F4 to set the Lab value obtained in the Lab coordinate system. In this case, the CMY value corresponding to each Lab value of the RGB→Lab3D−LUT is calculated from the CMY→Lab3D−LUT. Then the system goes to the Step F5 to search the CMY→Lab3D−LUT to find out the Lab value preset (inputted in advance). A decision step is taken to determine if this Lab value is located inside or outside the color gamut area of the CMY→Lab3D−LUT (color gamut area outside/inside decision step). If it has been determined in this decision step that the target of arithmetic processing Pin is located outside the printer color gamut, the system goes to Step F6 where the process of compression is carried out.
If it has been determined in the aforementioned Step F5 that the target of arithmetic processing pin is located inside the printer color gamut, the system goes to Step F7 where CMY value is found out by checking the CMY→Lab3D -LUT using the preset Lab value. The CMY value corresponding to this step of checking is calculated. Then the system goes to Step F8, where a decision step is taken to determine if the process of calculating the CMY value corresponding to the preset (inputted in advance) Lab value has been completed or not. If it has not yet completed, the system goes back to Step F4 and the aforementioned procedure is repeated. Upon completion of calculation of the CMY values for all the Lab values, the RGB→CMYK3D−LUT is created. Then the system goes to Step F9 where the RGB→CMY3D−LUT is converted into the RGB→CMYK3D−LUT by the GCR. This allows the RGB→CMYK3D−LUT to be created.
[Patent Document 1] Official Gazette of Japanese Patent Tokkai 07-236069
The conventional color image forming apparatus has the following problems:
i. In the color image forming apparatus to which the image processing apparatus disclosed in the Patent Document 1 applied, if the scanner color gamut is wider than the printer color gamut and the target of arithmetic processing (point e) is situated outside the printer color gamut, then the point e (the target of arithmetic processing) outside the printer color gamut is compressed inside the printer color gamut, as shown in FIG. 17B. Thus, when the input RGB value outside the printer color gamut has been converted by a color conversion interpolator, it will be inside the printer color gamut.
ii. In this connection, if the target of arithmetic processing outside the printer color gamut is interpolated inside the color gamut boundary, color difference may be further increased close to the color gamut boundary. Thus, effective use of the printer color gamut cannot be achieved in some cases.
iii. If the target of arithmetic processing outside the printer color gamut is interpolated inside the printer color gamut, unwanted colors may be mixed and color image quality may deteriorate due to color mixture.